r - Using mutate rowwise over a subset of columns

Question

I am trying to create a new column that will contain a result of calculations done rowwise over a subset of columns of a tibble, and add this new column to the existing tibble. Like so:

df <- tibble(
ID = c("one", "two", "three"),
A1 = c(1, 1, 1),
A2 = c(2, 2, 2),
A3 = c(3, 3, 3)
)

I effectively want to do a dplyr equivalent of this code from base R:

df$SumA <- rowSums(df[,grepl("^A", colnames(df))])

My problem is that this doesn't work:

df %>% 
select(starts_with("A")) %>% 
mutate(SumA = rowSums(.))
    # some code here

...because I got rid of the "ID" column in order to let mutate run the rowSums over the other (numerical) columns. I have tried to cbind or bind_cols in the pipe after the mutate, but it doesn't work. None of the variants of mutate work, because they work in-place (within each cell of the tibble, and not across the columns, even with rowwise).

This does work, but doesn't strike me as an elegant solution:

df %>% 
mutate(SumA = rowSums(.[,grepl("^A", colnames(df))]))

Is there any tidyverse-based solution that does not require grepl or square brackets but only more standard dplyr verbs and parameters?

My expected output is this:

df_out <- tibble(
ID = c("one", "two", "three"),
A1 = c(1, 1, 1),
A2 = c(2, 2, 2),
A3 = c(3, 3, 3),
SumA = c(6, 6, 6)
)

Best kJ

Answer 1

Here's one way to approach row-wise computation in the tidyverse using purrr::pmap. This is best used with functions that actually need to be run row by row; simple addition could probably be done a faster way. Basically we use select to provide the input list to pmap, which lets us use the select helpers such as starts_with or matches if you need regex.

library(tidyverse)
df <- tibble(
  ID = c("one", "two", "three"),
  A1 = c(1, 1, 1),
  A2 = c(2, 2, 2),
  A3 = c(3, 3, 3)
)

df %>%
  mutate(
    SumA = pmap_dbl(
      .l = select(., starts_with("A")),
      .f = function(...) sum(...)
    )
  )
#> # A tibble: 3 x 5
#>   ID       A1    A2    A3  SumA
#>   <chr> <dbl> <dbl> <dbl> <dbl>
#> 1 one       1     2     3     6
#> 2 two       1     2     3     6
#> 3 three     1     2     3     6

^{Created on 2019-01-30 by the reprex package (v0.2.1)}